linux-kernel - Re: [PATCH 3/3] net: ethernet: ti: am65-cpsw: Move phy_set_mode

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Date:   Wed, 1 Jun 2022 17:17:19 +0530
From:   Siddharth Vadapalli <s-vadapalli@...com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <vladimir.oltean@....com>,
        <grygorii.strashko@...com>, <vigneshr@...com>, <nsekhar@...com>,
        <netdev@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kishon@...com>
Subject: Re: [PATCH 3/3] net: ethernet: ti: am65-cpsw: Move phy_set_mode_ext()
 to correct location

Hello Russell,

On 01/06/22 15:25, Russell King (Oracle) wrote:
> On Wed, Jun 01, 2022 at 02:59:47PM +0530, Siddharth Vadapalli wrote:
>> Hello Russell,
>>
>> On 01/06/22 13:59, Russell King (Oracle) wrote:
>>> On Wed, Jun 01, 2022 at 11:39:57AM +0530, Siddharth Vadapalli wrote:
>>>> Hello Russell,
>>>>
>>>> On 31/05/22 17:25, Russell King (Oracle) wrote:
>>>>> On Tue, May 31, 2022 at 05:00:58PM +0530, Siddharth Vadapalli wrote:
>>>>>> In TI's J7200 SoC CPSW5G ports, each of the 4 ports can be configured
>>>>>> as a QSGMII main or QSGMII-SUB port. This configuration is performed
>>>>>> by phy-gmii-sel driver on invoking the phy_set_mode_ext() function.
>>>>>>
>>>>>> It is necessary for the QSGMII main port to be configured before any of
>>>>>> the QSGMII-SUB interfaces are brought up. Currently, the QSGMII-SUB
>>>>>> interfaces come up before the QSGMII main port is configured.
>>>>>>
>>>>>> Fix this by moving the call to phy_set_mode_ext() from
>>>>>> am65_cpsw_nuss_ndo_slave_open() to am65_cpsw_nuss_init_slave_ports(),
>>>>>> thereby ensuring that the QSGMII main port is configured before any of
>>>>>> the QSGMII-SUB ports are brought up.
>>>>>
>>>>> This sounds like "if we're configured via port->slave.phy_if to be in
>>>>> QSGMII mode, then the serdes PHY needs to be configured before any of
>>>>> the QSGMII ports are used". Doesn't that mean that if
>>>>> port->slave.phy_if is QSGMII, then the port _only_ supports QSGMII
>>>>> mode, and conversely, the port doesn't support QSGMII unless firmware
>>>>> said it could be.
>>>>>
>>>>> So, doesn't that mean am65_cpsw_nuss_init_port_ndev() should indicate
>>>>> only QSGMII, or only the RGMII modes, but never both together?
>>>>
>>>> The phy-gmii-sel driver called by phy_set_mode_ext() configures the CPSW5G MAC
>>>> rather than the SerDes Phy. In the CPSW5G MAC, the QSGMII mode is further split
>>>> up as two modes that are TI SoC specific, namely QSGMII main and QSGMII-SUB. Of
>>>> the 4 ports present in CPSW5G (4 external ports), only one can be the main port
>>>> while the rest are the QSGMII-SUB ports. Only the QSGMII main interface is
>>>> responsible for auto-negotiation between the MAC and PHY. For this reason, the
>>>> writes to the CPSW5G MAC, mentioning which of the interfaces is the QSGMII main
>>>> interface and which ones are the QSGMII-SUB interfaces has to be done before any
>>>> of the interfaces are brought up. Otherwise, it would result in a QSGMII-SUB
>>>> interface being brought up before the QSGMII main interface is determined,
>>>> resulting in the failure of auto-negotiation process, thereby making the
>>>> QSGMII-SUB interfaces non-functional.
>>>
>>> That confirms my suspicion - if an interface is in QSGMII mode, then
>>> RGMII should not be marked as a supported interface to phylink. If the
>>
>> CPSW5G MAC supports both RGMII and QSGMII modes, so wouldn't it be correct to
>> mark both RGMII and QSGMII modes as supported? The mode is specified in the
>> device-tree and configured in CPSW5G MAC accordingly.
>>
>>> "QSGMII main interface" were to be switched to RGMII mode, then this
>>> would break the other ports. So RGMII isn't supported if in QSGMII
>>> mode.
>>
>> Yes, if the QSGMII main interface were to be switched to RGMII mode, then it
>> would break the other ports. However, the am65-cpsw driver currently has no
>> provision to dynamically change the port modes once the driver is initialized.
> 
> If there is no provision to change the port mode, then as far as
> phylink is concerned, you should not advertise that it supports
> anything but the current mode - because if phylink were to request
> the driver change the mode, the driver can't do it.
> 
> So, you want there, at the very least:
> 
> 	if (phy_interface_mode_is_rgmii(port->slave.phy_if))
> 		phy_interface_set_rgmii(port->slave.phylink_config.supported_interfaces);
> 	else
> 		__set_bit(PHY_INTERFACE_MODE_QSGMII, port->slave.phylink_config.supported_interfaces);
> 
> which will still ensure that port->slave.phy_if is either a RGMII
> mode or QSGMII.

Thank you for reviewing the patch. I will send v2 for this series implementing
the fix suggested above.

Thanks,
Siddharth.